How to run script when a monitor is connected/disconnected?

[dangh]

Hello,

I have a second monitor that I put below the main monitor. I want to run a script to change the Dock setting (dock to the left when it's connected, otherwise dock to the bottom). Is it possible to do so with BetterDisplay?

[waydabber]

Sadly BetterDisplay can't move the dock around, it's simply not in the feature set currently. 😞

[dangh]

@waydabber No I can move the dock using a shell script. I just need to trigger the script when the monitor on connect/disconnect.

[waydabber]

Oh, I see. Well, you can check if a display is connected or disconnected via BetterDisplay's CLI but there is no way currently to trigger a script based on events. There are some tools like HammerSpoon that have something like that or you can simply create a very simple solution with some coding of your own that looks out for the changes by looking for the right system notification dispatch and check the nature of the change.

For more info:
https://developer.apple.com/documentation/coregraphics/1455336-cgdisplayregisterreconfiguration

[waydabber]

Note: this simple ChatGPT prompt created the right code for me:

Please create a simple Swift code that creates a macOS terminal app that runs in the background and looks for displays connected or disconnected and launches a shell script whenever such an event happens. Use CGDisplayRegisterReconfigurationCallback for watching the right event.

import Foundation
import CoreGraphics

// Function to be called when a display configuration changes
func displayReconfigurationCallback(_ display: CGDirectDisplayID, flags: CGDisplayChangeSummaryFlags, userInfo: UnsafeMutableRawPointer?) {
    if flags.contains(.addFlag) {
        print("Display connected")
        runShellScript()
    } else if flags.contains(.removeFlag) {
        print("Display disconnected")
        runShellScript()
    }
}

// Function to execute the shell script
func runShellScript() {
    let scriptPath = "/path/to/your/script.sh" // Change this to the actual path of your script

    let task = Process()
    task.executableURL = URL(fileURLWithPath: "/bin/sh")
    task.arguments = [scriptPath]

    do {
        try task.run()
        task.waitUntilExit()
    } catch {
        print("Failed to run shell script: \(error)")
    }
}

// Register the callback
CGDisplayRegisterReconfigurationCallback(displayReconfigurationCallback, nil)

// Keep the app running in the background
RunLoop.main.run()

(note: did not test the result but at first glance looks like what I would have done)

[dangh]

Thank you for pointing me to Hammerspoon. I have it working now.